A concise answer (without trying to be flippant) is "the past"; I get the majority of my hardware out of a dumpster. Since it's stuff that typical computer users have thrown out, it tends to be older and has had time for open-source support to be developed if possible. Similarly, bought stuff is often clearance, which is a bit older. This explains why I don't have any wireless AC gear. Generally with bought stuff, I'll try to look around the web for reports of success with Linux at a minimum before buying.

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If I'm not mistaken, Linux supports AC Wi-Fi by now more or less, so why not BSD? Is there less interest, or the drivers are closed? I have found some, seemingly open-source friendly AC Wi-Fi dongles as well.

I'm not really sure about the state of AC on linux since I haven't needed it, but it wouldn't surprise me if there's usable support by now. As for why not BSD, I'd guess the usual multiple reasons like lack of funding and person-power, licensing, etc. Many of the BSD manual pages for wireless N gear will say they only are supported in G mode. So AC support would seem rare/surprising. I also don't recall reading of AC support on this forum, which doesn't prove there is none, but sort of concurs with the general picture.